Notes:
Summary A brush border membrane fraction from rat renal cortex binds the polyvalent protease inhibitor (Trasylol®) from bovine organs. Saturation is complete in the presence of about 40 nanomoles/ml of protease inhibitor. The maximal binding capacity is about 18 nanomoles (or 117 μg) of protease inhibitor per mg of brush border membrane protein. The affinity constant, as calculated from the double reciprocal plot of binding, is 7 μM. Removal of the microvillar knobs by incubation with papain does not change the binding characteristics, whereas incubation of the brush border with sialidase markedly decreases the fixation of the peptide. Guanidination fails to alter the binding properties, whereas the tetramaleoyl derivative is not fixed at all. Therefore, the positive net charge of the peptide seems to play an important role. Insulin, glucagon and bradykinin interact with the brush border fraction in a manner entirely different from that of the protease inhibitor. Over a wide concentration range, about 6 to 10% of these peptides are bound independent of the concentrations added. Our present and previous results suggest that binding to the brush border membrane is the initial step in the pinocytotic reabsorption of the protease inhibitor and possibly of other peptides too. The tremendous concentration of the protease inhibitor by the kidney cortex is assumed to be due to its high affinity to the brush border membrane in vivo.